Leaching of columbium and tantalum spalt



United States Patent LEACHING OF COLUMBIUM AND TANTALUM SPALT Filed June 20, 1958, Ser. No. 743,481

7 Claims. (Cl. 75-101) No Drawing.

This invention relates to an improved process for the purification of tantalum and columbium metals. More specifically, the invention pertains to a process for treating tantalum and columbium metals produced by the reduction of their double fluorides.

In many of the processes proposed for the production of tantalum and columbium metals, the double fluoride salts of the metals are separately recovered and then subjected to either chemical or electrolytic type reduction to obtain the respective metals. The steps of separating tantalum and columbium from each other, the preparation of the double fluoride salts of these metals, and the reduction of the salts to produce the metals are generally known to the art and do not form a part of this invention. Following reduction of the double fluoride salts, it has been customary to Wash or leach the reduction product mixture, often referred to as spalt, in order to remove undesirable by-products and impurities from the columbium and tantalum metals. Such treatments have included leaching with water and/or mineral acid solutions, e.g. aqueous hydrochloric acid. In British Patent No. 791,121, for example, the reaction product mixture containing columbium, obtained by the reduction of potassium fluocolumbate with sodium, was first leached with water and then washed with demineralized water to obtain a yield of about 90% columbium. Bars of columbium prepared from this product had a hardness of 130 to 150 V.P.N. (140 to 160 B.H.N.).

In utilizing the prior art methods for recovering the tantalum and columbium metals from the reduction product mixtures numerous problems have been encountered. For one thing, at least 10% of the metal could not be recovered, and the finished metal had high hardness values. In order to devise an effective commercial process, conditions must be found whereby high quality tantalum and columbium metal fines can be successfully leached and high quality metals can be recovered even from incompletely reduced spalt.

It is one object of this invention to provide a leaching or washing process which avoids the difficulties encountered when utilizing the prior art methods. Another object of the invention is to provide a process whereby the percentage of metal recovered is increased with the recovered metal having improved hardness values. Further objects of the invention will become apparent from the ensuing description of the invention.

In accordance with the present invention, it has been found that the use of buttered leaching or wash solutions resulted in increased yields and metal products having improved hardness values.

These achievements are of 2 considerable importance for commercial operations. Though the exact manner in which the buffered solutions aid in achieving these results is not fully understood, it is believed that several undersi'r-able side-reactions occurring in the prior art methods are avoided. In order to explain this in detail, the treatment of a reduction product mixture or spalt obtained by the reduction of potassium fluocolumbate with metallic sodium will be described.

The reduction product mixture will, in general, contain the following constituents:

(.1) Columbium metal (2) Potassium fluoride (3) Sodium fluoride (4) Free sodium (trace) (5) Potassium fiuocolurrrbate (1(2CbF7) It will be understood that other materials, such as alkali metal halides, may be present in the spalt. When this reduction product mixture is leached with Water; sodium hydroxide is formed from the reaction between the free sodium and the water. The sodium hydroxide will in turnreact with the K CbF to produce Cb(OH) and CbO(OH) as Well as Na CbO which are insoluble in water and acids and will contaminate the columbium sponge or powder product causing an undesirable increase in hardness. On the other hand, leaching with an acid solution, e.g. HCl, has the disadvantage in that free hydro fluoric acid is produced by the reaction between the hydrochloric acid and the sodium and potassium fluorides. The hydrofluoric acid in this mixture will react with the columbium metal to form hydrogen and soluble HgCbFq. The soluble H CbF will dissolve in the leach liquor and be lost or, if acidity is decreased, will hydrolyze to insoluble hydroxide and yield brittle metal. In the early stage of leaching where KF concentration is high, the HgCbFq will form KgCbFq which will precipitate as the hydroxide with decreasing acidity and clog up the pores of the metal with'the resulting hydroxide making complete fluoride removal impossible. It has been determined that even. a local or short term unfavorable condition will cause hard metals. It will be understood, of course, that the treatment of reduction product mixtures containing tantalum, columbium or mixtures thereof will involve analogous problems. Moreover, it is obvious that if any of the above undesirable side reactions are allowed to proceed, the amount of metal recovered and its purity will be aflected.

As set forth above, it has now been discovered that these problems can be resolved by utilizing a suitable buffer system in the leaching or washing solutions. Examples of eifective aqueous buffers include the following:

(a) Sodium acetate-acetic acid (b) Ammonium acetate-hydrochloric acid (0) Ammonium dihydrogen phosphate (d) Ammonium chloride-hydrochloric acid (e) Sodium oxalate-oxalic acid (1) Sodium citrate-citric acid, etc.

In general, any buffer system which will maintain a pH of about 5.0 to 6.2, preferably about 5.4 to 6.0, in the leaching solutions'rnay be employed in the process of this invention. It is especially advisable to'maintaina pH of Patented Nov. 15,1960

about to 6, preferably about 5 .4 to 5.7, when leaching a tantalum spalt and a pH of about 5.6 to 6.2, preferably about 5.8 to 6.0, when leaching a columbium spalt.

Any of the known methods for preparing bufler systems may be employed for the present purposes. Stock solutions of the buffer may be prepared, for example, by simply mixing the components together in water. In general, about 50 to 100 ml. of the bufier will be employed per 1000 ml. of leach solution.

The invention will be more fully understood by reference to the following illustrative examples:

EXAMPLE I Run A.--About 1.37 mols of KzTaFq were reduced with 186 grams of metallic sodium, constituting a sodium excess of about 5%, in a stainless steel reactor under argon at a maximum temperature of 1000 C. The resulting reaction product mixture was sintered for 8 hours at 800 C. The spalt so obtained was ground to an average particle size of about 12 mesh and then leached with an aqueous hydrochloric acid solution (pH 3) until a test for soluble fluorides was negative. The spalt was next leached with distilled water to remove the final traces of chlorides. The tantalum metal recovered weighed 220 grams (theoretical yield-246 grams); metal recovery was, therefore, 89.5%. The tantalum metal was brittle, and it had an oxygen content of 1.06%.

Run B.A sodium acetate buffer was prepared by dissolving 358 grams of sodium acetate and 250 grams of glacial acetic acid in enough water to make 1000 cc. of stock solution. For use in this run, 55 cc. of the stock solution were diluted to 1 liter with water. A spalt was prepared by reducing K2TOF7 with 1% excess metallic sodium. The tantalum spalt was ground to a particle size of about 12 mesh, and leached with the 1 liter portion of the aqueous solution containing the buffer until no test for fluorides was obtained with calcium chloride. The tantalum metal so recovered was filtered, washed with distilled water and then dried. The entire process took about 4 to 6 hours. The recovery was 98% of theoretical, and the metal had a Brinell Hardness of 65-70.

EXAMPLE II Table A Runs M1. of H01 (6 N) added to 66 g. of

N HAOAC in 3,000 cc 45 90 180 270 Final pH of salt-containing solution"- 5. 5 5. 7 5. 6 5. 4 4. 2 Brinell Hardness No 95 64 100 107 286 From the above data is appears that tantalum metal of good hardness, 100 B.H.N. or less, can be obtained by this system over a wide range of buffer compositions. Moreover, the recovery of tantalum was 98-99% of theoretical, when the hardness was about 100 B.H.N.

EXAMPLE HI Utilizing the same procedure as in Example II, 124 grams of columbium spalt were leached with ammonium 4 acetate-hydrochloric acid buffer systems. tabulated below:

The results are Table B Runs Ml. of H01 added to 66 gms. of

NH4OAO in 3,000

cc 0 ll 23 34 45 180 Final pH of saltcontainingsolutlou. 6.90 6.65 6.18 o. 89 5.50 5.00 6. 30 BEN Brittle 157 103 217 235 Brittle The above data also indicate that in leaching a columbium spalt more rigid control over pH must be maintained than in the case of tantalum spalt.

While certain embodiments of this improved process have been shown and described to illustrate the broad aspects of this invention, it will be understood that various modifications and changes may be made in this process as indicated to those skilled in the art without departring from the spirit of this invention. Thus, for example, potassium fluotantala-tes and fluocolumbates may be reduced with various reducing agents such as sodium, potassium, magnesium, aluminum, etc., to give the spalts which can be leached or washed according to the inventive process.

What is claimed is:

1. A method for leaching a material selected from the group consisting of tantalum spalt, columbium spalt, and mixtures thereof; said tantalum spalt containing tantalum, alkali metal fluorides, alkali metal and an alkali metal fluoride of tantalum, and said columbium spalt containing columbium, alkali metal fluorides, alkali metal and an alkali metal fluoride of columbium, which consists of leaching said material with a leach solution selected from the group consisting of water and mineral acid solution, said leach solution having a buiier added thereto to maintain in said leach solution a pH of 5.0 to 6.2.

2. The method of claim 1 wherein said pH is within the range of about 5.4 to 6.0.

3. The method of claim 1 wherein said material is tantalum spalt and said pH is within the range of about 5.4 to 5.7.

4. The method of claim 1 wherein said material is columbium spalt and said pH is within the range of about 5.8 to 6.0.

5. A method for leaching a material selected from the group consisting of tantalum spalt, columbian spalt, and mixtures thereof; said tantalum spalt containing tantalum, alkali metal fluorides, alkali metal and an alkali metal fluoride of tantalum, and said columbium spalt containing columbium, alkali metal fluorides, alkali metal and an alkali metal fluoride of columbium, which consists of leaching said material with a leach solution selected from the group consisting of water and mineral acid solution, said leach solution having a butter selected from the group consisting of sodium acetate-acetic acid, ammonium acetate-hydrochloric acid, ammonium chloridehydrochloric acid, sodium oxalate-oxalic acid, and ammonium dihydrogen phosphate, added thereto to maintain in said leach solution a pH of 5.0 to 6.2.

6. The method of claim 5 wherein said buiier is sodium acetate-acetic acid.

7. The method of claim 5 wherein said butter is ammonium acetate-hydrochloric acid.

Fowler Sept. 13, 1949 McCord May 26, 1959 

1. A METHOD FOR LEACHING A MATERIAL SELECTED FROM THE GROUP CONSISTING OF TANTALUM SPALT, COLUMBIUM SPALT, AND MIXTURES THEREOF, SAID TANTALUM SPALT CONTAINING TANTALUM, ALKALI METAL FLUORIDES, ALKALI METAL AND AN ALKALI METAL FLUORIDE OF TANTALUM,AND SAID COLUMBIUM SPALT CONTAINING COLUMBIUM, ALKALI METAL FLUORIDES, ALKALI METAL AND AN ALKALI METAL FLUORIDE OF COLUMBIUM, WHICH CONSISTS OF LEACHING SAID MATERIAL WITH A LEACH SOLUTION SELECTED FROM THE GROUP CONSISTING OF WATER AND MINERAL ACID SOLUTION, SAID LEACH SOLUTION HAVING A BUFFER ADDED THERETO TO MAINTAIN IN SAID LEACH SOLUTION A PH OF 5.0 TO 6.2. 